1. Field of the Invention
The present invention relates to device and method for coding an image, and more particularly, to device and method for coding an image, in which wavelet transformed sub-bands are scanned in horizontal or vertical blocks suitable for the sub-band characteristic.
2. Background of the Related Art
There is the wavelet transform as one of transform methods for coding a video signal, which is means similar to the Fourier Transform for disassembling a signal in a time (space) domain into respective frequency components. The wavelet transform improves a locality of the time domain in a high frequency region and a locality of a frequency domain in a low frequency region.
FIG. 1 illustrates a structure of a sub-band region when a video signal is wavelet transformed.
Referring to FIG. 1, it is well known that, as a result of subjecting a video signal to a wavelet transform, a structure divided into sub-band region becomes to have a low frequency region which contains relatively more meaningful information on the video signal and is concentrated on one side thereof, and a high frequency region which contains relatively less meaningful information on the video signal and is concentrated on the other side thereof. That is, low frequency components of an original video is gathered to the most coarse band (F0) and fine high frequency components are gathered to bands (F1xcx9cF9) other than the coarse band. All the coefficients for given bands except the lowest frequency band have relations with coefficients for the next sub-band in a similar direction. That is, of the nine sub-band regions F1xcx9cF9 in the wavelet transformed sub-band regions shown in FIG. 1, F1, F4 and F7 regions show horizontal edge components of the video signal, and F2, F5 and F8 regions show vertical edge components, and F3, F6 and F9 regions show diagonal components. Therefore, if the wavelet transformed respective sub-band regions are scanned in different paths (horizontal, vertical and diagonal) by using such a characteristic, a more effective video compression can be made. That is, as shown in FIG. 2, by scanning the F1, F4 and F7 regions which have horizontal edge components in a horizontal direction, the F2, F5 and F8 regions which have vertical edge components in a vertical direction, and the F3, F6 and F9 regions which have diagonal edge components in a diagonal direction, a continuous edge portion scan can be made. And, by doing so, a better compression ratio can be obtained in an arithmetic coding conducted later owing to a statistical characteristic (a probability is increased, in which consecutive zero runs are occurred by scanning in a direction that shows the edge components).
However, a necessary and satisfactory compression ratio may not be obtained only by scanning each subband in directions proper to the subband characteristic. That is, as shown in FIG. 3, if the F7 regions is reviewed in detail, it can be known that even if a horizontal edge component F7a also has many pixels in a vertical direction. Therefore, if the subband F7 is simply scanned in a horizontal direction, there will be a zero run which is not continuous occurred at an end portion of a horizontal edge, that causes a reduction of the compression ratio than expected in an arithmetic coding in a later stage.
Accordingly, the present invention is directed to device and method for coding an image that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the method for coding an image for compressing a digital video signal, includes the steps of (1) transforming a spatial domain of the video signal into frequency domain, and dividing the frequency domain into subband regions having relatively much meaningful information on an original video signal and subband regions having no relatively much meaningful information on the original video signal, (2) classifying the divided subband regions into a plurality of scan blocks each having a block size, and (3) scanning in a scan block unit, whereby increasing a probability of consecutive zero run statistically, to improve the compression ratio.
A transform, such as wavelet transform, is applied to the video signal for transforming the video signal into an appropriate frequency domain.
In other aspect of the present invention, there is provided a device for coding an image including transforming means for transforming a spatial domain of an input image into a frequency domain, and classifying the frequency domain into subband regions having a variety of resolution, scan controlling means for classifying the classified subband regions into a plurality of scan blocks each having fixed sizes, and scanning means for scanning in the scan block unit.
The scan controlling means adaptively applies a width:length size of the scan block depending on an edge component of the subband region each scan block belongs thereto being in a horizontal direction, a vertical direction, or diagonal direction.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.